Generally, a large-sized transporter vehicle called a dump truck has a liftable vessel (loading platform) on a frame of a vehicle body. The large-sized transporter vehicle carries objects to be transported such as crushed stones or earth and sand in a state where the objects to be transported are loaded in a large quantity on the vessel (for example, Japanese Patent Laid-Open No. 2006-264423 A).
A transporter vehicle of this type according to the conventional art comprises an automotive vehicle body, a vessel which is provided on the vehicle body in such a manner as to be tiltable (liftable) thereon by using a rear side of the vehicle body as a fulcrum and on which objects to be transported are loaded, an operating room provided in the vehicle body to be positioned on the front side of the vessel, and the like. A steering handle is provided in the operating room of the transporter vehicle, and a driver grips the steering handle to steer a travelling direction of the vehicle.
The transporter vehicle of the conventional art is equipped with a power steering apparatus constructed to perform a steering operation of a vehicle using a hydraulic force for being able to assist a handle operation of a driver. This power steering apparatus is constructed by including a steering valve for controlling a direction and a flow amount of pressurized oil supplied to a steering cylinder at the time of being changed from a neutral position to a changeover position by a steering handle, a circulation line for circulating an oil liquid in the steering valve when the steering valve is in the neutral position.
In this case, a stationary orifice is provided in the steering valve to be positioned along the path of the circulation line. This stationary orifice applies a throttling function to the oil liquid circulating in the steering valve when the steering valve is in the neutral position to generate a frictional heat for keeping the steering valve to be always in a warm state, thus protecting the steering valve from a so-called heat shock.